Not applicable.
Not applicable.
Not applicable.
This invention is directed to methods and apparatus for treating cellulose materials with preservatives, and more particularly methods and apparatus for heating and replenishing a preservative compound during treatment of cellulose materials in order to better distribute and concentrate the preservative within the cellulose materials and the like.
The need to treat cellulose materials, such as wood and wood products with preservatives and fire retardants is a very old need that continues to remain unsatisfied. For example, for centuries, a preservative to protect the hulls of wooden ships from damage and possible destruction through marine borers-the Teredo worm-was sought through copper sheathing of the hulls"" wetted surfaces or through application of various preservative paints, all with greater or lesser effectiveness. Until recently, it had been customary to protect wood and wood products (e.g. chip board, plywood, railroad ties, dock pilings and the like) from exposure through creosote treatment. Creosote treatment was a reasonably satisfactory preservative, but its use is being discontinued because the environmental effects of creosote are unacceptable.
Accordingly, a need still exists for an economical and environmentally acceptable wood and wood product preservative and fire retardant treatment process. Toward this end, it has been found that silicates protect cellulose materials at least to some extent, from insects, the destructive effects of fungus, bacteria and the like. These silicate preservatives have been applied to the cellulose material in several ways. Frequently, the silicates were painted on the material to be preserved; the material was immersed in a silicate solution; or the material was enclosed in a pressure vessel and immersed in a hot silicate solution under pressure to enable the silicate to invade the material being preserved and to permeate its cellulose structure.
These processes for applying a silicate, however, were not entirely satisfactory. Painting or immersing the material in a silicate solution resulted in only a superficial coating of preservative that failed to provide not only a durable, long term treatment, but left untreated those insects, fungus and bacteria that were within the material structure at the time of treatment. Pressure treatment also failed to produce a fully acceptable finished product. Thus, a charge of preservative was mixed with water and heated to form a hot solution in a pressure vessel that contained the material to be treated. The hot solution was pressurized and left in contact with the material for a period of time that was determined by the nature of the product and the preservative.
The solution cooled rapidly to ambient temperature, thus decreasing process efficiency. The preservative compound in the solution also became depleted when the preservative penetrated and was absorbed within the material being treated. This preservative depletion was a particularly serious defect because those portions of the material under treatment that were exposed only to the depleted solution were inadequately treated and hence, were essentially unprotected. Frequently, the preservative compound also would settle out of the entraining water, thereby reducing the quantity of preservative available for direct application to the cellulose matter being treated.
Consequently, there is need for a process and apparatus for applying preservatives to cellulose material that establishes a better distribution of preservative throughout the cellose matter being treated and deposits the preservative in a concentration that is high enough to protect the material more completely. That is, in a concentration sufficient to eliminate or at least to arrest cellulose deterioration from insect, fungus, bacteria and other sources.
These and other deficiencies in the prior art are overcome, to a great extent through the practice of the invention. Illustratively, cellulose material, such as stack of railroad ties is placed in a pressure vessel. The loaded pressure vessel is sealed and a vacuum is drawn in the pressure vessel. About the same, a time work tank is filled with a charge of wood preservative and water. The mixture is heated and stirred throughly in the work tank. On reaching the appropriate temperature, a valve is opened and the heated liquid flows into the evacuated pressured vessel. A pump raises the liquid pressure within the pressure vessel to a predetermined level above that of atmospheric pressure whereupon the liquid penetrates the physical structure of each of the ties that comprise the stack. As the preservative is deposited within the structure of each of the ties, the concentration of preservative within the liquid becomes depleted.
Thus, in accordance with a feature of the invention, depleted liquid is pumped from the pressure vessel back to the work tank where it is reheated prior to recirculation from the work tank to the pressure vessel in order to maintain thermal efficiency for the process and, through recirculation, to keep entrained preservative materials in suspension. At the work tank, if the preservative in the liquid is sufficiently depleted, more preservative is mixed with the liquid and as noted above, this mixture, is heated to the process temperature. The heated and replenished liquid then is pumped into the pressure vessel in order to subject the stack of railroad ties to a preservative solution at full potency. The replenishment of preservative and circulation of heated liquid is then carried out continuously in the foregoing manner until the stack of railroad ties has been suitably treated.
This new process produces a treated material that enjoys not only a much more uniform distribution of preservative, but also a higher and therefor more effective concentration of preservative within the cellulose structure than that which characterized prior art treatment processes.